The invention relates to a method for the attachment of components to a circuit board comprising applying a photoactivatable adhesive to selected areas of the components or of the circuit board, fitting the components to the circuit board, curing the adhesive bonds at a temperature between 60.degree. and 40.degree. C. and producing soldered joints between the components and the circuit board.
Such a method has been proposed in the German Patent No.2,904,649, in which "surface mounted technology (SMT)" is used to attach electronic components such as capacitors, resistors and the like to circuit boards, as for instance printed circuit boards, hybrid ICs and the like using a photoactivatable adhesive, the bond serving primarily to hold the components on the circuit board firmly until the soldered joints between the components and the circuit board have been produced.
In this known method UV curable resins are used as adhesives, which contain a proportion of polymerizable monomers, photo-sensitizers, initiators and possibly a series of fillers.
Such UV curable acrylate resins rapidly cure at the parts which are accessible to light, but however for the curing at sites in the shadow, that is to say more especially where they are covered by the components, they require a second curing mechanism, which is effected by initiators for thermal polymerization and requires high temperatures. However, the greater the thermal load on the electronic components, the shorter their statistical expectancy of life.
Attempts have already been made to reduce the thermal loading during thermo-setting of the adhesive composition by using two adhesive points for each component with half under and half directly adjacent to the component in order in this manner to increase the degree of cure in the first UV induced cure stage. Integrated circuits (chips) with pins arranged around the housing do not however have any free space available for such half drops of adhesive.
The UV curable adhesives used in known methods are furthermore sensitive to atmospheric oxygen. Furthermore the pronounced coloring of the drop of adhesive often required for chip adhesives may reduce the depth of penetration of the UV radiation, and thus prevent a complete cure and later lead to signs of corrosion.
The European Patent publication (EP-A) No.0 182 744 describes photocurable epoxy resin compositions containing fillers, which have at least one photoinitiator of the metallocen type and which inter alia may be used for the bonding of active and passive components in microelectronic systems on circuit boards providing that the compositions contain electrically conducting fillers. The UV induced photocure is then if needed followed by a subsequent thermal cure.
The post published European Patent publication (EP-A) No.0 279 199 describes a method and apparatus for processing UV curable reactive resin compositions, in which the UV curable reactive resin composition is irradiated in the application device and is supplied directly to a substrate or a casting mold. There is no account of a method for the attachment of components to a circuit board however in this publication. Similarly the Japanese Patent publication (JP-A) No.6198740 describes the production of molding and coating compositions using a UV preactivated resin matrix. However, there is again no mention of any attachment of components to a circuit board.